In the paper entitled “Measurement Report: Molecular composition, sources, and evolution of atmospheric organic aerosols in a basin city in China”, the authors analyzed the atmospheric PM_2.5 in Chengdu during winter season, especially using GC/MS determined the molecular composition of OA. Firstly, they conducted a analysis of the chemical composition of PM_2.5 and OA during the observation period; Secondly, they conducted a detailed introduction of the concentration, chemical composition, and sources of OA species. Finally, based on the chemical composition and source apportionment results, they analyzed the characteristics of pollution evolution and proposed the potential implications of this study. As a whole, the logic of the manuscript is reasonable and clear, and a large number of observational results are reported. These pieces of information have reference value for understanding OA in Chengdu and even the Sichuan Basin. However, I have found that there are still some issues in the current manuscript that need to be addressed or clarified, such as Section 3.2 currently contains too many details but appears to be less important information, which needs to be simplified. Therefore, I think a minor revision is necessary before considering acceptance.

1 Introduction. I think the first two paragraphs need to be rewritten. Especially the effects of PM_2.5 and OA mentioned by the authors seem to be repetitive.

2 Section 2.1. Please provide data sources on the population and motor vehicle ownership in Chengdu city.

3 Section 2.3. Reference on “Interagency Monitoring of Protected Visual Environments thermal evolution (IMPROVE) protocol” is necessary.

4 Line 137. The authors have provided an explanation regarding the data, namely “mean ± standard deviation”. Therefore, the “average” before the data is unnecessary. Similar modifications are also required for other parts of the manuscript.

5 Line 148. “OC” in the Figure title should be changed to “OM”.

6 Line 155. I suggest the author increase the discussion on the contribution of inorganic components, especially in comparison with previous research results.

7 Line 224-225. “…between fatty acids and the biomass burning and fossil fuel combustion…”. This sentence needs to be expressed more clearly.

8 Line 253-255. “Although levoglucosan dominated the total sugars in our study, its contribution (57.5%) is lower than that reported 10 years ago in urban areas of China (up to 90%) (Wang et al., 2006) ” . Is this comparison for the same city? As discussed by the authors, differences in the types of biomass burning in different regions may also lead to variations in the chemical composition of anhydrosugars.

9 The discussion about “primary saccharides” can be simplified.

10 Line 308-309. The standard deviation of the average relative humidity should be provided.

11 The authors attribute the high concentration of phthalate esters during the daytime to high temperature, however, in other sections they stated that the temperature increase in daytime did not cause an increase in the concentration of OA species, which seems contradictory. I think a reasonable explanation is necessary.

12 Line 417. Please provide the full names of “4NC and 4NP”.

13 Line 534. Is the PMF source analysis result for winter? Generally speaking, the seasonal differences in source composition are significant. Comparing with other seasons can be misleading.

14 Line 595, “relatively stable” is an ambiguous term, and a more accurate expression is necessary.

15 Regarding the evolution characteristics of pollution. The authors found that during the process of worsening pollution, the contributions of secondary inorganic and SOC both increased significantly. However, the contributions of two types of SOA, namely SOAs and OPAHs, have decreased. What is the reason for this contradictory phenomenon? Is it due to differences in formation mechanisms?

This study gives a very detailed measurement report on molecular composition, sources, and pollution evolution mechanisms of organic aerosols (OA) in PM_2.5 during winter in a big city Chengdu, Sichuan Basin, China. The methods and analysis are robust with results clearly presented. As the molecular-level OA research for this region is still very limited, this study is helpful understanding the OA formation mechanism in this region and providing  insights for air pollution control measures. I recommend the publication of this study after a minor revision.

Minor comments:

(1) Add one sentence in the end of the abstract clarifying the significance or implication of this study.

(2) The use of GC/MS to quantify fatty acids, phthalate esters, and anhydrosugars etc, is well-established. However, for some more volatile organic compounds (e.g., isoprene SOA tracers), could the authors clarify potential losses of these relatively high volatility compounds during derivatization and describe correction measures?

(3) For the PMF model, the factor of plastic-related sources is interesting. Could the authors include specific markers, e.g., styrene derivatives or tire-wear tracers, to strengthen the credibility of this factor?

(4) Add references for Lines 357 – 360 about the formation mechanisms of OPAHs and its role in formation of ROS.